The present invention relates to devices and methods for improving hearing, particularly in the field of hearing aids. The invention is an output actuator that is a component of a class of hearing devices known as surgically implantable hearing aids. This invention relates to both fully implanted and partially implanted hearing aids. More particularly, methods and devices are disclosed to provide an actuator for directly driving the inner-ear fluid, or the middle-ear bones referred to as the ossicular chain, resulting in the sensation of hearing.
Over 26 million people in the United States suffer from some type of hearing loss. A large portion of this population can regain the ability to hear or at least improve their diminished hearing with the use of a hearing aid. Yet, many people choose not to use a hearing aid for such reasons as social stigma, the discomfort associated with a device in the ear canal, the unnatural, hollow sound and/or plugged up sensation that some hearing aid users report (commonly referred to as the occlusion effect), and noise caused by feedback of the device. Surgically implantable hearing aids address all of these concerns and could increase the frequency of use by those individuals previously reluctant to use hearing aids. A detailed discussion on the usefulness and benefit of implantable hearing aids is found in U.S. Pat. No. 5,772,575 to Lesinski et al.
Like most natural processes of the body, the ability to hear is made possible by an intricate process involving many steps. The mechanical portion of this intricate process takes place in the outer ear, middle ear, and the inner ear. The outer ear, the auricle, collects sound waves and leads these waves into the middle ear. The middle ear couples the sound waves in the air-filled ear canal to fluid of the inner-ear (perilymph). The middle ear, containing the eardrum (tympanic membrane) and three tiny bones (malleus, incus and stapes), is an interface between the low impedance of air and high impedance of inner ear fluid. Pressure induced vibrations of the tympanic membrane ultimately induce a proportional motion of the stapes, the smallest of the three auditory ossicles in the middle ear. This motion is the output of the middle-ear. The stapes transmits this motion to the inner ear. In the inner ear, this motion produces a large pressure in the scala vestibuli, a perilymphatic channel on one side of the cochlear duct, in comparison with the scala tympani, a perilymphatic channel on the other side of the cochlear duct separated from the tympanic cavity by the round window membrane. The pressure difference between the two scalae in turn causes a traveling wave to move apically on the basilar membrane. The motion of the basilar membrane causes the cilium of receptor cells, also known as the inner hair cells (IHC) to move, which in turn causes firing of the auditory nerve. This process produces the sensation of hearing.
The ability to hear and the sensitivity at which one is able to hear is diminished by two basic types of ear pathologies that are commonly referred to as i) conductive hearing loss, and ii) sensory-neural hearing loss. Conductive hearing loss may be traced to either a pathological condition of the middle ear or the middle-ear cavity, or impairment (i.e., blockage) of canal or the outer ear. This type of hearing loss is routinely repaired by otologic surgeons. On the other hand sensory-neural hearing loss is due to a pathological condition of the inner ear and is nearly impossible to repair via surgery.
Different pathological conditions of the inner-ear can lead to sensory-neural impairment. See, for example, Killion, M. C. (1997) xe2x80x9cSNR Loss: I can hear what people say but I can""t understand them,xe2x80x9d The Hearing Review 4(12)8-14 (1997). First, there is the loss of outer hair cells (OHC), normally organized in three to four rows along the length of the basilar membrane. In this condition there is a decrease in basilar membrane motion and consequently there is a reduction in movement of the receptor cells. Most researchers agree that loss of OHC results in an increase in threshold to tonal stimuli. That is, the loss of OHC appears to reduce an individual""s ability to hear quiet or low volume sounds. The loss of inner hair cells (IHC) or their cilium (hair bundles) is another disease state of the inner ear. It is believed that IHC provide all of the auditory information to the brain. Thus, in this pathological state, there is a decrease in the number of auditory nerve fibers that send neural impulses to the more central portion of the auditory system. As a result, as seen with loss of OHC, the loss of IHC results in an increase in threshold to tones. In addition, it has been speculated that loss of IHC also causes a loss of clarity of hearing. In other words, it is thought that loss of IHC results in an effective increase in internal noise and thus requires a greater signal-to-noise ratio (SNR) than patients with no IHC pathology (Killion, 1997). In this type of hearing loss there is a reduction in an individual""s ability to understand speech (i.e., the signal) in the presence of background sound (i.e., the noise). By itself, any hearing aid can address the threshold issue and will improve an individual""s ability to hear quiet or low volume sounds. Yet, not all hearing aids will address the signal-to-noise ratio issuexe2x80x94i.e., most hearing aids fail to improve one""s ability to hear speech in the presence of background noise.
Two commonly found causes of sensory-neural hearing loss are presbyacusis and noise induced hearing loss. Presbyacusis is the loss of ability to perceive or discriminate sounds. This loss of high frequency hearing increases with age. Hearing is also compromised by an individual""s exposure to loud sounds. For example, without hearing protection, sounds from machinery, excessive live or recorded music, gun shots, etc. cause sensory-neural hearing loss. The extent of damage depends upon the intensity, frequency, content, and duration.
Individuals having a high degree of sensory-neural hearing impairment, but who still have some residual hearing capability, can achieve normal pure-tone thresholds if the motion of the stapes is amplified. In other words, exaggerating the motion of the stapes permits a hearing impaired individual to hear sounds that were previously too soft to hear. Alternatively, driving the cochlear fluid by other means (e.g., at a location other than the stapes), and at an amplified level, also improves the ability of the hearing impaired to hear sound. Basically, the location of where cochlear fluid is put into motion does not matter. This phenomena is known as xe2x80x9cparadoxical motionxe2x80x9d and was described by the Nobel laureate Von Bekesey (1960). It is this xe2x80x9cparadoxical motionxe2x80x9d that is the basis for bone-conduction hearing which is routinely measured in audiology clinics.
Several individuals have proposed methods for directly driving cochlear-fluid. See, e.g., Yanagihara, N., Gyo, K., Suzuki, J., and Akara, H. (1983). xe2x80x9cPerception of sound through direct oscillation of the stapes using a piezoelectric ceramic bimorph,xe2x80x9d Ann Otol Rhinol Laryngol 92:223; Yanagihara, N., Suzuki, J., Gyo, K., Syono, H., and Ikeda, H. (1984). xe2x80x9cDevelopment of an implantable hearing aid using a piezoelectric vibrator of bimorph design: State of the art,xe2x80x9d Ann Otol Rhinol Laryngol.; and Suzuki et al., Middle Ear Implant for Humans, Acia Otolaryngol (Sockh) (1985) 99:313-317. The entirety of the above references is hereby incorporated by reference. These documents describe output transducers for use in implantable hearing aids. These hearing aids rely upon a piezo bimorph. A bimorph consists of two piezo materials bonded together, sometimes having a metallic sheet (a shim) sandwiched between the piezo materials. The bimorph causes bending deformation as each piezo material produces extension or contraction under an electric field. The bonding of the two materials allows for a magnification of the displacement that is otherwise obtainable. These documents describe a piezo bimorph that is anchored to bone at one end of the bimorph. The other end of the bimorph is attached to the head of the stapes footplate. Sensation of hearing is demonstrated by applying an electrical signal to the bimorph. The functional gain achievable with a bimorph transducer depends on the length of the transducer. Because of the limited space in the middle ear, the functional gain of the Yanagihara output transducer is limited. Also, a drawback common with bimorphs includes low response speed and low generative force due to the bending mode of the materials. Although the shim increases the reliability of the piezo by maintaining structure if the piezo materials fracture, the shim adds to the size of the transducer.
U.S. Pat. No. 5,277,694 to Leysieffer et al., describes processes for driving the cochlear fluid by methods such as driving the stapes directly (as discussed by Yanagihara et al.), or by a piston through a hole made in the footplate of the stapes. At the heart of this patent is a piezo disk that sits on flexible membrane. Radial motions of the piezo causes the membrane to move, thereby causing motion of the inner-ear fluid.
U.S. Pat. No. 5,411,467 to Hortman et al. proposed an electromechanical converter. The transducer is a piezo that separates two fluid-filled chambers. One chamber has a tube that acts as a hydromechanical coupling element to the inner ear.
U.S. Pat. No. 5,772,575 to Lesinski et al. describes an actuator placed in the scala tympani through the promontory, or near the round window. In one embodiment, the transducer is fabricated from a thin circular disk of stress-biased unimorph PLZT material. This transducer is attached to a thin membrane to provide a simply supported structure and fluid-seal the entire transducer assembly. As in the Hortman et al. patent, the actuator output is coupled to the inner ear with a tube.
More recently, U.S. Pat. No. 5,707,338 to Adams et al. discusses placing a transducer on the stapes footplate itself. In Adams et al., sound is transmitted to the inner-ear fluids by flexing of stapes bone. That is, a vibration produced by the transducer causes a deformation of the footplate, thereby vibrating the inner-ear fluid. This approach causes large deformations of the footplate and resultant fractures in the footplate bone which lead to leakage of perilymph into the middle-ear cavity. Leakage of perilymph compromises an individual""s ability to hear. In an another embodiment, the head of the stapes is removed thereby disarticulating the ossicles, and a perforation is made in the stapes footplate (as in a stapedotomy procedure). A bi-element transducer is then placed where the head of the stapes was cut. A rod is inserted between the footplate hole and the transducer to transmit motions of the transducer to the cochlear fluid. Disarticulating the stapes has the disadvantage of eliminating any residual natural hearing.
U.S. Pat. No. 5,772,575 to Lesinski et al., teaches the use of an implantable microactuator and implantable microphone to create vibrations in the perilymph fluid within a subject""s inner ear, and U.S. Pat. No. 5,857,958 to Ball et al. teaches the use of a floating mass transducer that may be implanted or mounted for producing vibrations in a vibratory structure a subject""s ear. The entirety of both patents is hereby incorporated by reference.
As shown above, many of the existing devices used for driving the stapes or inner ear fluid rely upon piezo actuators. Upon the application of an electrical potential, a piezo material expands and contracts. This is the classical electrical-to-mechanical piezo-electric effect first described by Pierre and Jacques Curie. Published in 1880, the Curie brothers were first to demonstrate the experimental connection between macroscopic piezoelectric phenomena and crystallographic structure. The most important measure of functionality of a piezo is the dmn coefficient that specifies mechanical motion in the n-axis for an applied E field in the m-axis of the transducer. Commonly, the d33 coefficient is along the thickness of the transducer, while d31 and d32 are orthogonal to the d33 constant. For an applied field in a given direction the sum of the displacement must be zero, since the volume of the solid must remain constant.
One limitation found in the current methods for driving the stapes or the inner ear fluid is attributable to the limit of suitable available space in the middle ear cavity. The bones of the middle ear are quite small. Likewise the middle ear cavity itself is quite small. Therefore, there exists a need to find a compact method and/or device to drive the mechanics of the middle or inner ear. Current methods to drive the ear using piezo transducers yield limited gain due to limitations on maximum applied voltages, or to physical dimensions. There remains a need for an improved hearing aid that overcomes the limitations described above.
The invention herein relates to hearing aids using a piezo in the flextensional modes to produce hearing enhancement. Flextensional transducers have existed since 1920""s and have been used as underwater transducers since the 1950""s. Flextensional devices typically consist of a piezoelectric element sandwiched between two specially designed metal-shell, or plastic-shell, end caps. The end caps mechanically transform the radial motion of the piezo disk into a large axial displacement normal to the surface of the end caps. The shape of the shell to a large extent determines the mechanical advantage. These transducers are described in numerous publications [eg., Tressler, Newnham and Hughes (1999), JASA 105: 591-600]. For a more thorough discussion of flextensional transducers, see U.S. Pat. No. 5,729,077 to Newnham et al., the entirety of which is hereby incorporated by reference.
As discussed in more detail below, this invention is an implantable hearing aid using a flextensional transducer. For a piezo in the flextensional modes, as described herein, the d31 and d33 coefficients of the piezo element contribute to an amplified displacement of the inventive transducer in the desired axial direction. The inventive transducer may drive the perilymphatic fluid of the inner-ear directly or may drive the stapes or the footplate. The substrate comprises a piezo. In the current invention, a single-crystal piezo (SCP) is preferred, but the invention does not exclude the use of other types of ferroelectric material such as poly-crystalline ceramic piezos, polymer piezos, or polymer composites.
This invention relates to devices and methods relating to implantable hearing aids for placement within a middle ear or the inner ear. In particular, the invention includes at least one output actuator comprising a piezo substrate typically having a first and a second substantially planar surfaces, a thickness, and a transverse size. The substrate changes in thickness when a voltage is applied to the material. The substrate may be, but is not limited to, a single crystal piezo (SCP). Also, the substrate may be a single layer or may be a multi-layer composite. Alternatively, the substrate may be dome-shaped.
Another variation of the invention includes a composite substrate comprising a plurality of substrate components. The substrate components are aligned such that the composite substrate has a thickness, a first and a second substantially planar surfaces, and a composite transverse size.
The output actuator also has a first end cap mounted on a planar side of the substrate, the cap having an actuating surface. The first end cap may be fixedly attached to a portion of the substrate in a manner such that a change in the transverse size of the substrate causes the actuating surface of the cap to move in a direction orthogonal to the surface of the substrate. The output actuator is generally, but not necessarily encased within a biocompatible material. The output actuator is also in mechanical communication with an auditory component of the middle ear such as an ossicle, or fluid of the inner ear.
The output actuator may also have a second end cap mounted on a planar side of the substrate opposite the first end cap. This second end cap also has an actuating surface. The second end cap may be fixedly attached to a portion of the substrate in a manner such that a change in the transverse size of the substrate causes the actuating surface of the cap to move in a direction orthogonal to the surface of the substrate.
The implantable hearing aid may also comprise output actuators which are stacked in a series. The output actuators may be placed at the incudo-stapedial joint, in which case the actuator may be an inverted cymbal design. The output actuator may have end caps having contoured shapes which accommodate or fit the incus and the head of the stapes.
Another variation of the invention includes placing an output actuator in mechanical communication with an auditory component of the middle ear. For example, an output actuator may be attached to a stapes. In this variation, the actuator may be located adjacent to the head of the stapes and to the incus. It is further contemplated that the actuator may be placed either on the footplate of the stapes or in a hole in the stapes. In another variation, the caps may be bowed towards the substrate material. As with the above variation, it is a variation of the invention to include a spacer comprising an expandable flexible portion with the output actuator.
Another variation of the invention includes placing an output actuator between an incudo-stapedial joint between a stapes and an incus. This variation may include having end caps of the actuator shaped to receive a head of the stapes and/or an incus. Another variation of this output actuator includes using an inverted-cymbal output actuator.
In another variation of the invention, the output actuator may be placed in contact with a footplate of the stapes. The actuator may also be placed in an artificial hole made in the footplate of the stapes. In such a case, the hole may be lined with a membrane that may consist of either a piece of vein, fascia, or adhesive. Another variation is that the output actuator has an end cap having a size larger than that of the hole. In such a case the larger end cap rests against the footplate of the stapes while the remaining portion of the actuator is placed within the hole in the footplate.
The output actuator may be round or of a prismatoid shape. As mentioned above, the prismatoid shape takes advantage of the anatomical configuration of the footplate of the stapes, e.g., the footplate is longer in one direction than the other. The end caps of the actuator may be made of a superelastic alloy, a metal alloy, or a polymeric material. Typically, the size of the output actuator is less than 5 mm but the actuator is not limited to this dimension.
In another variation of the invention, the implantable hearing aid may be configured to be implantable in the inner ear. It is contemplated that an output actuator may be placed directly into contact with the inner ear fluid. Alternatively, the output actuator may be placed within an assembly that has a portion adapted for rigid insertion into a bony portion of the promontory. It is another variation that the actuator may have a single or double end caps.
As noted above, the end caps of the output actuator may be made from a superelastic alloy, a metal alloy, or a polymeric alloy.
Another variation of the invention is a spacer having a mounting portion and having a shape conforming to a portion of an auditory component and a flexible portion adjacent to the mounting portion. The flexible portion has a compressed state, a natural state, and an expanded state. The spacer can expand from the natural or compressed state into the expanded state upon reaching a temperature substantially near to body temperature. The change in shape is preferably due to the use of a shape-memory alloy which expands at a temperature near the body temperature. A variation of the spacer includes a mounting portion that has a shape conforming to a portion of a stapes within the middle ear. The spacer may have a flexible portion that is configured to receive an output actuator. The spacer maybe made from superelastic or shape memory alloys.
In one variation, the spacer is positioned between the auditory component and the output actuator. Once the spacer approaches body temperature, the spacer secures the output actuator to a desired location as it expands against the output actuator.
In another variation of the invention, an ossicular attachment may be configured for attachment to an incudo-stapedial joint. In this variation the output actuator is placed between a head of the stapes and the incus. As with the other variations, the output actuator may have contoured end caps to accommodate and fit the head of the stapes and the incus.
In another variation, the implant may be configured for mechanical stimulation of the fluid within the inner ear. The implant may either directly stimulate the fluid within the middle ear or it may directly stimulate an intermediary fluid which is hydraulically coupled to inner ear fluid but separated from the inner ear fluid by a membrane.
Yet another variation of the invention includes a method of improving hearing comprising the steps of providing at least one output actuator as generally defined herein, providing a voltage to the substrate to change the traverse size of the substrate to produce a proportional movement in an actuating surface, and positioning the actuator in communication with a portion of the ear to directly transmit the movement of the actuating surface to the portion of the ear.
A variation of the inventive method includes placing the actuator in contact with a stapes, a footplate of the stapes, or a hole in a footplate of the stapes. The actuator may also be placed in contact with the incudo-stapedial joint. The actuator may also be placed in fluid communication with the fluid of the inner ear or in a vestibule fluid space.
Any of the features of one variation of the invention may be combined into or with another variation of the invention.